If you're determined to go that route, you'll need something much thicker. Most heatsink backing plates are ribbed for more rigidity - the ones that are flat are often using anywhere from 8-14 gauge metal. Your best bet is to find a metal-mart of some sort and see if they sell smaller pieces. You'll also need some sort of tool for cutting the stuff - a hacksaw will take a long time. Also make sure it won't short out the back of the motherboard - a few layers of rubber/neoprene/felt cloth will make sure of that.